912 BULLETIN OF THE BUREAU OF FISHERIES. 



with a thin layer of sediment. I maintain that thus the embryos also suffered, 

 and I am sure that it is in this way the fry contract the disease of the gills. 



Such is the origin of the disease which manifests itself in young fisji in 

 swollen gills, impregnated with dirt and bulging so much as to push out the 

 operculum. The lish are seen to weaken and turn dark in color and then die, 

 at first in small numbers, but increasing to an almost complete mortality. It 

 may be justly assumed that those which escape are those which hatched last 

 and have not been so long in contact with the deposit on the bottom of the 

 trough. This terrible disease may not manifest itself among the young fish 

 until the age of five to six weeks. 



In addition to the cause I have named for the origin of this disease of the 

 gills, it is sometimes due to placing too nmch food into the rearing troughs. 

 The coagulated blood, spleen pulp, pulp of liver, etc., becoming diluted, render 

 the water at times so tinted that it is impossible to see through it. The gills 

 are impregnated with putrefiable matter, which soon gives rise to this terrible 

 epidemic, especially if the water is not aerated and frequently changed. 

 Lastly, a great number of fry in a small space, not receiving a sufficient quantity 

 of water, acquire the disease of the gills by atrophy, as may be easily recognized 

 by the small size and emaciation of the fish. 



MEASURES OF PREVENTION. 



As stated at the outset, the remedy for this disease is prevention, which 

 involves the utmost care on the part of the attendant, with also the form of equip- 

 ment which shall permit of the most perfect cleanliness. I will indicate the 

 means I have employed and consider essential. 



1. Discard all apparatus that does not allow the interior to be examined. 

 It must be possible to see under the grilles on which the eggs are placed, and, if 

 necessary, to clean the bottom of the trough without interfering with the 

 incubation process. 



2. Use an equipment which will permit of retaining the fry therein after 

 the hatching is over. Discard the wooden grille frame for one of metal and 

 replace the glass tubes with solid glass rods. 



3. Have an ample supply of water in proportion to the number of eggs in 

 the trough, and create currents to reach every part. Increase the supply of 

 water, if possible, as soon as the eye spots appear in the egg. 



4. Do not crowd the eggs on the grille. Take out the dead eggs each day. 

 Do not refill the spaces left by the removal of dead eggs. These, taken out to 

 prevent their bursting in the trough, will now give more space to the developing 

 eggs. Take out any eggs containing monstrosities. 



5. Give a thorough cleaning both to grilles and troughs when the eggs are 

 ready to hatch, so that the fry may fall to the bottom of the trough without 

 danger to their gills. A hand siphon cleaner" should be used for the trough. 



a Vincent E.: Devices for use in fish hatcheries and aquaria. Proceedings Fourth International 

 Fishery Congress, Bulletin U. S. Bureau of Fisheries, vol. xxvm, 1908, p. 1030. 



